Evaluating open magnetic structures such as rods and bobbins is less clear cut than closed magnetic structures. Since the majority of the field is not contained within the core, the initial permeability of the material has a greatly reduced influence on the inductance of the wound assembly therefor, the permeability of the material is more difficult to evaluate. The purpose of this tool is to aid in being able to identify and cross reference the material a rod type core is made out of. This tool will calculate effective permeability (µrod) of the rod-type core in question when the following values are entered: rod diameter, rod length, number of turns wrapped around the diameter of the rod, length of the winding (measured in the same direction as the length of the rod), and the measured series inductance of the wound rod. The tool will calculate a rod permeability value which can be used comparatively against the accompanying chart to find a best fit material.
The main characteristic differentiating Fair-Rite’s material grades is initial permeability. This is the inductive portion of the complex permeability of a material measured specifically at 10kHz with a flux density of less than 10 gauss (sinusoidal excitation). Calculating the initial permeability on the sample in question is the most reliable way to get an approximation of its material grade. To evaluate the permeability, you will need insulated wire and some way to measure inductance with a 10kHz sinusoidal signal (in a pinch 1kHz-100kHz will yield a similar number for most materials). This tool will calculate the permeability of a toroidal type core when the following values are entered by the user: three critical mechanical dimensions (outer diameter, inner diameter, part height), the number of passes through the aperture of the core, and the measured series inductance of the wound core. The resulting permeability number can then be matched up to a ferrite material by comparing the mi value published. Keep in mind the having an exact match to a published mi Value is unlikely due to material variation and measurement equipment differences.
Utilization of bobbin ( spool or drum) cores is a simple and cost effective way of designing a power inductor. This calculator makes use of standard bobbins in Fair-Rite’s # 77 material – so frequency is limited to about 10 MHz, depending on the winding. Maximum possible NI is 750 Ampere-Turns, maximum current is 15 Amps ( using AWG# 10) and minimum wire gauge is AWG# 30. The user enters target inductance and maximum operating current : the first thing the calculator does is selects a minimum wire gauge based on 700 cir-mils/Amp ( 2.8 sq.mm/A) Current Density. Then the calculator estimates number of turns required and selects the smallest size bobbin core capable. Uncoated bobbins, magnet wire and 90% copper fill ratio are assumed.
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